Perhpas some get help by iOS: Troubleshooting FaceTime but its pretty generic.
Facetime has stopped working for me since IOS 6.1 (I think), on facettime calls. If you start off with a cell call (so iPhone not iPad) and then reqeust/accept factime it seems to connect significantly better, but was still complaining about poor network quality (despite both ends having excellent connectivity) and dropped the call a few times. Starting out making a factime call with iPhone or iPad has yet to work at all. both ends just get "connecting ....." until it gives up.